The present invention relates to a process for preparing expandable particles of a vinyl chloride resin composition usable for beads-foaming.
As processes for preparing foamed articles of a vinyl chloride resin composition, there have hitherto been known (a) a process in which a chemical blowing agent which gasifies by heat decomposition is admixed with a vinyl chloride resin and the mixture is heat-molded to foamed articles by an extruder or an injection molding machine; (b) a process in which a vinyl chloride resin and a plasticizer are mixed to prepare a plastisol, and the plastisol is mechanically foamed, or a chemical blowing agent is admixed with the plastisol and the mixture is melted and foamed by heating; (c) a process in which a mixture of a vinyl chloride resin and a chemical blowing agent is previously roll-molded at a temperature of not higher than the decomposition temperature of the chemical blowing agent and then foamed by heating; (d) a process in which a mold is filled with a vinyl chloride resin and a chemical blowing agent (if necessary, an easily volatile organic blowing agent and an organic solvent which swells or softens the resin are additionally used), heated under pressure to melt, cooled down, and heated again to foam; (e) a process in which an extruder is charged with a vinyl chloride resin into which a volatile blowing agent is previously impregnated or an extruder is charged with a vinyl chloride resin and then a blowing agent is introduced into the extruder to carry out extrusion foaming; and the like.
However, the above-mentioned processes (a) to (e) have the following various defects.
By the processes (a) to (c), rigid or semi-rigid foamed articles having high expansion ratios cannot be prepared; as to the process (d), the costs of end products are great since the process is a batchwise process of which producing steps are complicated and a lot of time is required for preparing the foamed articles; and as to the process (e), though rigid or semirigid foamed articles having high expansion ratios can be relatively easily prepared, foamed articles having complicated shapes cannot be prepared due to extrusion foaming.
Vinyl chloride resin foamed articles have various advantages such as good noninflammability, good toughness and low cost over polystyrene foamed articles. However, there have not been provided expandable particles of vinyl chloride resins which can be molded in high expansion ratios by beads-foaming technique.
In view of the above-mentioned defects of the conventional processes for preparing foamed articles of a vinyl chloride resin composition and in order to prepare easily foamed articles having complicated shapes, the present inventors made a study of a process in which melting and pelletization of a specific vinyl chloride resin were carried out, the pellets were impregnated with a volatile blowing agent and heated to make pre-expanded particles, and the pre-expanded particles were molded in a mold. By this process, though pre-expanded particles were prepared, foamed articles having desired shapes could not be made by heat-molding in a mold. That is, when the pre-expanded particles were molded at a low temperature, they did not fuse together and therefore the molded article broke when it was taken out from the mold, and when the pre-expanded particles were molded at a high temperature, they melted and returned to their original pellets. After a further in-depth study, the present inventors have found that foamed articles having desired shapes can be prepared only when specific resin compositions and specific blowing agents are combined.
An object of the present invention is to provide a process for preparing expandable particles of a vinyl chloride resin composition which can make foamed articles having desired shapes.
The above and other objects of the present invention will become apparent from the description hereinafter.